There's nothing that says that a servo has to be a DC motor, or a particular number of wires.
As stated on the Wikipedia articles I linked to...
What you are describing is a hobbyist RC servo. The qualifications are important!
"Servo" means any actuator drive that has a sensor, feedback loop, and controller. There are lots of servos that don't use a potentiometer, instead using higher-quality sensors (optical, magnetic, etc.) There are lots of servos that don't have a three-wire wiring harness. There are lots of servos that don't come with horns. There are lots of servos that use AC motors.